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Now a judge has ordered him released from prison and granted a new trial

"I am in here for murder," he told CNN in 2013. "A murder I would not, could not have done."

Sycamore, Illinois (CNN)The man convicted in the oldest cold case ever brought to court broke into a wide grin as a judge ordered him released from prison Friday and granted him a new trial.

Jack McCullough, 76, was serving a life sentence for the 1957 murder of 7-year-old Maria Ridulph. Judge William Brady threw out the conviction after a prosecutor found "clear and convincing evidence" that McCullough was wrongly found guilty.

He was freed shortly after the hearing and was whisked away in a car driven by a family member.

After the judge's ruling, McCullough turned toward his stepdaughter, Janey O'Connor, and flashed her a private signal that she says means "I love you." She was seated behind him and broke into tears but managed a big smile back.

Read the original series on the case: "Taken"

Maria Ridulph's murder went unsolved for half a century. Then detectives pursued a tip, and a man was brought to trial and convicted in the 1957 murder of the 7-year-old in Sycamore, Illinois. Now that man is free. Ann O'Neill's 2013 series on the case, "Taken," raised questions about whether the trial was unfairly one-sided.

Across the aisle, Charles Ridulph, the older brother of the victim, frowned and bowed his head. More than 58 years ago, he helped search for Maria after she vanished from a street corner near their home while playing in the snow with a friend.

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McCullough's lawyers and DeKalb County state's attorney Richard Schmack argued that McCullough's conviction was based on false testimony, improper legal rulings controlling the evidence presented, and a timeline that was tweaked some 50 years after the fact to rule out McCullough's alibi.

McCullough has long insisted that he couldn't possibly have abducted and killed the child because he was 40 miles away in Rockford, Illinois, talking to recruiters and trying to enlist in the U.S. Air Force when she was taken.

He repeated the alibi when he spoke with CNN in prison in March 2013.

"They proved nothing," he said at the time. "I am in here for murder. A murder I would not, could not have done."

McCullough said that when Maria was kidnapped, the telephone in his home in Sycamore was ringing.

"And I was on the other end of that phone, in Rockford, three minutes before she was kidnapped. Try and make that happen. Only Scotty could make that happen, if he beamed me up."

Jack McCullough, in prison after his 2012 conviction. Now a judge has ordered him freed.

An Illinois appeals court upheld his conviction last year. But McCullough made a last-ditch appeal in a jailhouse motion last December, saying police and prosecutors buried evidence supporting his alibi. He asked a judge to find him innocent.

Schmack, who inherited the case from predecessor Clay Campbell, was placed in the position of having to defend the conviction. He launched a six-month investigation that included a review of some 4,500 pages of documents -- old police and FBI reports, grand jury transcripts, trial transcripts, affidavits for search and arrest warrants, and even CNN's five-part series on the case, "Taken," which raised questions about whether the courtroom reconstruction of history was unfairly one-sided.

Schmack concluded that he'd found "clear and convincing evidence" that McCullough had been convicted of a crime he didn't commit.

The precise time of Maria's abduction has been in dispute almost from the beginning.

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The dark-haired second-grader with big brown eyes was with her playmate, Kathy Sigman, on December 3, 1957 when they were approached by a stranger who said his name was Johnny. He gave Maria two piggyback rides, Sigman told police at the time. Sigman, now a grandmother known as Kathy Chapman, said she went home to fetch her mittens. When she returned, Maria and Johnny were gone.

More than 600 volunteers combed the fields and woods around Sycamore in search of Maria, at times walking hand-in-hand in a grid pattern. And within 24 hours, dozens of FBI agents descended on the small farming community about 65 miles west of Chicago.

Agents interviewed some 200 possible suspects -- including McCullough, who passed a polygraph test about his alibi and was cleared. Despite the work of nearly three-dozen FBI agents, the investigation turned up no solid leads to Maria's whereabouts.

Her body was discovered 144 days later by a couple looking for mushrooms near the Iowa border. She was under a fallen tree, some 100 miles from home.

Because Maria apparently never left Illinois, the FBI pulled out of the investigation and the case went cold for half a century.

The Illinois State Police began to look into the Ridulph case again in 2008, after McCullough's half sister, Janet Tessier, emailed a tip. She said her dying mother had pulled her close some 14 years earlier and whispered, "Those two little girls, and the one that disappeared, John did it. John did it, and you have to tell someone."

McCullough was questioned in Seattle and arrested in late June 2011. A judge hearing the case without a jury found McCullough guilty after a week-long trial.

Schmack, the state's attorney, said he sat through the trial and always doubted whether there was enough evidence to support the conviction. In his review of the case, he said he found new evidence that verified McCullough's long-standing alibi.

He subpoenaed records from AT&T and found that a collect call had indeed been placed at 6:57 p.m. to McCullough's home in Sycamore from a pay phone at the post office in downtown Rockford on on December 3, 1957 -- just as McCullough said he had done.

At his sentencing, McCullough vehemently professed his innocence.

"Look in the box. The truth is in the box," he said, pointing to a cardboard box in the courtroom. It was filled with old FBI reports and other documents that the judge, James Hallock, kept from the trial, saying it was inadmissible hearsay.

Prosecutor Richard Schmack reviewed the case and asked for McCullough to go free.

Schmack did look at those documents. He built a timeline based on the statements of 21 people -- Maria Ridulph's family members, friends, neighbors and passersby -- that confirmed the FBI's original timeline ruling out McCullough as a suspect.

Maria was taken that day some time between 6:45 and 7 p.m. -- and not as early as 6 p.m., as alleged during McCullough's trial.

It would have been impossible for him to have been in two places at once.

Campbell, the state's attorney who brought the case, was defeated by Schmack in an election just weeks after McCullough was convicted. Campbell has declined to comment since Schmack released his findings.

Cold cases are particularly difficult to prosecute because evidence is often lost or destroyed, memories fade and witnesses die. And so questions have lingered over the evidence used to convict McCullough -- and whether it was strong enough.